DataSource that routes to one of various target DataSources based on the
current transaction isolation level. The target DataSources need to be
configured with the isolation level name as key, as defined on the
TransactionDefinition interface.

This is particularly useful in combination with JTA transaction management
(typically through Spring's JtaTransactionManager).
Standard JTA does not support transaction-specific isolation levels. Some JTA
providers support isolation levels as a vendor-specific extension (e.g. WebLogic),
which is the preferred way of addressing this. As alternative (e.g. on WebSphere),
the target database can be represented through multiple JNDI DataSources, each
configured with a different isolation level (for the entire DataSource).
The present DataSource router allows to transparently switch to the
appropriate DataSource based on the current transaction's isolation level.

The configuration can for example look like this, assuming that the target
DataSources are defined as individual Spring beans with names
"myRepeatableReadDataSource", "mySerializableDataSource" and "myDefaultDataSource":

Alternatively, the keyed values can also be data source names, to be resolved
through a DataSourceLookup: by default, JNDI
names for a standard JNDI lookup. This allows for a single concise definition
without the need for separate DataSource bean definitions.

Note: If you are using this router in combination with Spring's
JtaTransactionManager,
don't forget to switch the "allowCustomIsolationLevels" flag to "true".
(By default, JtaTransactionManager will only accept a default isolation level
because of the lack of isolation level support in standard JTA itself.)